Overflow

In the context of roof drainage, overflow is a device that relieves the primary drainage system from the excess volume of rainwater during the intense rainfall or reaches the maximum design intensity.

The rainwater drainage codes in many countries require the provision of secondary systems as overflow drainage.

According to BS8490:2007, overflow is as a device installed in a gutter or flat roof that discharges the flow if the water level rises above a limiting level (e.g. due to exceptional rainfall or blockages at outlets). All roof drainage systems should be designed to minimize the risks of water entry into the building in the event of extreme rainfall conditions or the effects of blockages. Overflows or emergency outlets should be provided on flat roofs with parapets and in non-eaves gutters in order to reduce the risk of over spilling of rainwater into a building or structural overloading. Primary and Secondary system should operate independently. Secondary system shall deal with more intense storms up to the maximum design intensity for the scheme. This can be achieved by preventing flow from entering the outlets of the secondary system until water levels in the gutter or flat roof exceed a certain limit.